Many frothy beverages like milk beverages, chocolate beverages, . . . are often prepared by mixing a food soluble powder or a food liquid concentrate with a diluent. Mixing devices are known for speedier preparation of such beverages by mixing the soluble food component with the diluent, such as water. These devices typically comprise a mixing chamber in which the soluble component and the diluent are fed. The diluent can be introduced into the dissolution chamber in order to create a whirlpool to efficiently dissolve the soluble component in the hot water or the hot diluent can be introduced under the form of jets that provides mixing, dissolving and frothing. The mixture is then usually frothed by a rotor in a whipping chamber to reconstitute the beverage and produce foam. The beverage is then usually evacuated from the whipping chamber through the bottom of the chamber and dispensed into a receptacle for drinking.
In the prior art different types of whipping chambers have been developed for improving the quality of the foam of beverages. EP 1 476 060 and EP 1 639 924 describe similar whipping chambers in which the rotors present a tapered shape. Grooves or ribs are present on the rotors surfaces whereas the back wall of the whipping chamber can presents protrusions or ribs. These whipping chambers were conceived for frothing various soluble powder ingredients; in particular these chambers were configured for frothing milk beverages as well as coffee beverages depending on the speed of actuation of the rotor. Consequently these whipping chambers are not particularly optimized for the frothing of milk. Besides due to the presence of fine grooves on the rotor and ribs on the back wall these whipping chambers cannot be easily rapidly cleaned.
WO 2003/005868 describes a whipping chamber wherein the internal wall surface of the chamber comprises indentations. Due to these indentations cavitations are created when the rotor is activated. The rotor can also present breakers. This whipping chamber can be indifferently used for the preparation of lattes, cappuccinos or espressos and consequently this whipping chamber is not particularly optimized for the frothing of milk.
WO 2008/013452 describes a whipping chamber wherein the internal wall of the chamber comprises ribs in particular near the outlet of the chamber so as to maintain the beverage in the chamber as long as possible and avoids that instant powder lumps are present in the final beverage. This dispenser induces a longer time of dispensing of the beverages since the beverage is maintained in the whipping chamber as long as the rotor is actuated. It means that the customer has to wait for a longer time before having his beverage totally dispensed in the drinking cup. For example for the preparation of a cappuccino beverage which requests first the dispensing of a frothy milk and then the dispensing of a coffee in the frothy milk, the sum of the time for frothing the milk, then for dispensing the milk with the same whipping chamber induces a too long dispensing time for the customer. Again this whipping chamber is used for the preparation of various different beverages in particular soup, coffee, hot cocoa, tea, lemonade and fruit juices and is not particularly dedicated to the preparation of milk beverages with a high quality of foam.
Today there is an increasing demand of the customers for specialty beverages based on milk and especially based on frothed milk like cappuccino, latte macchiato or cocoa based beverages. The properties of the milk foam of these beverages play an important role in the quality of the final beverage shall it concerns the visual aspect or the mouthfeel of the beverage. In particular these beverages can be well prepared if the milk foam presents the following specific properties:                the volume of foam must be sufficient. For example, in a latte macchiato recipe one third of the beverage volume must be composed of milk foam. It is then important to get a high ratio of milk foam compared to milk liquid.        the foam must be dense. It means that it must look like a microfoam with a very tiny bubble size. This quality is important when a layered beverage is produced such as a latte macchiato because dense foam helps for slowing down the flow of coffee and preventing said flow from falling down to the bottom of the drinking receptacle, which must remain white for the layering effect. Besides a dense white foam is essential for realising latte art on the top of the beverage.        the foam must present an homogeneous aspect with bubbles of uniform small size. This quality participates to the nice visual aspect of the foam and to the nice mouthfeel.        the foam must present a shiny appearance for the visual aspect,        the foam must be creamy. This property is linked to the degree of stiffness of the foam which must remains sufficiently liquid and not rigid. This can be reached by incorporating a lot of liquid in the foam which means producing very small bubbles to get a smaller air/liquid ratio.        the foam must be stable so that the foam remains present all along the time during which the customer drinks his beverage; the foam must keep its initial properties all along this time.        
The qualities of the milk froth are different from those required for other frothed beverages like coffee espressos. The whipper chambers of the prior art have provided a good compromise being able to produce either a correct espresso crema or a correct milk froth with the same device. But none of the whipper chambers of the prior art has been able to provide a real high quality foamed milk.
The object of the present invention is to propose a mixing device dedicated to the foaming of milk in order to provide a high quality milk foam for the preparation of frothed milk beverages.